1. Field of the Invention
The present invention relates, in general, to resistance spot welding apparatus and methods and, specifically, to resistance spot welding electrode caps.
2. State of the Art
In resistance spot welding operations on metal parts, electrode assemblies including- electrode arms, shanks and replaceable electrode caps are movably mounted with respect to each other so as to contact the metal parts and supply electric current therethrough to form resistance spot welds. The end diameter or the contact surface of the electrode cap which contacts the metal part is one of the critical factors in forming a quality, high strength spot weld.
In manufacturing operations, it is common for a single welding gun to perform numerous spot welding operations. Due to the high contact forces involving in such welding operations, the end diameter or contact surface of the electrode caps gradually enlarges or "mushrooms". This increases the end diameter or contact surface and thereby decreases the current density as a spot weld is formed in the metal parts. If the end diameter becomes too large, current density will fall below a critical value and result in a low strength, weak weld.
Thus, it is necessary for the electrode caps to be frequently replaced with new caps having the proper end diameter or dressed manually by use of a file or other grinding implement to restore the end diameter to the proper size. Both replacement and dressing are labor intensive which creates additional problems besides the need for a skilled maintenance repairman. Due to typical high production rates, there is often not enough time between each welding gun cycle for a maintenance repairman to replace or manually dress the electrode caps on a welding gun. Thus, such repair is frequently postponed until normal production down time. However, due to the constant, gradual enlarging or mushrooming of the electrode caps, this delay in replacing or dressing the caps to the proper size could result in numerous low strength, weak welds which directly affects product quality.
If the electrode caps are replaced or manually dressed on the welding gun at the required time during production, the time required to replace or dress the electrode caps could result in an interruption of the production line thereby effecting hundreds of workers and lowering production rates.
In an effort to minimize the manual labor required to replace or dress welding electrode caps and to alleviate the possibility of production stoppages, an automatic electrode cap exchanging apparatus has been proposed in U.S. Pat. No. 4,794,221. In this apparatus, a knife tool is mounted on one end of a shaft rotatably mounted on a base. A swinging lever is connected to one end of the shaft to rotate the shaft about its axis. The knife tool engages an annular groove or recess formed at the end of the electrode shank between the main portion of the electrode shank and the electrode cap. Rotation of the knife tool by activation or rotation of the shank generates a twisting action in the knife tool which separates and removes the electrode cap from the electrode shank.
This patent also describes the use of an electrode cap holder containing two adjacent rows of outwardly facing new electrode caps which are located adjacent to the knife tool. A robot welder which carries the welding electrode assembly moves the electrode assembly from the knife tool to the holder. The electrode shanks of the welding gun are then activated and moved toward each other to forcibly engage new electrode caps in the holder.
Although this electrode cap exchanging apparatus eliminates the need for manual labor in replacing electrode caps and minimizes the possibility of production stoppages or down time due to the fast operating cycle of the apparatus and the robot welder, it contains design features which could prevent its widespread use on most commonly employed welding electrode assemblies. The welding electrode assembly employed by the electrode cap changing apparatus in this patent is specifically designed to create an annular groove or recess between the inner end of the electrode cap and an adjacent shoulder on the electrode shank. This provides a readily accessible space for the knife tool to engage the electrode shank. However, a large number of electrode assemblies employed in certain industries, such as the automobile industry, utilize a welding electrode design which does not have an annular groove between the inner end of the electrode cap and the shoulder on the electrode shank. In these welding electrode configurations, the inner end of the electrode cap is disposed in registry or contact with the shoulder on the electrode shank. Since the knife tool disclosed in the above-described patent moves only through a rotation or twisting action, it does not have sufficient force to engage the joint between the registered electrode cap end shank so as to separate the electrode cap from the shank in these particular electrode assembly configurations.
Thus, it would be desirable to provide a resistance spot welding electrode cap changer which automatically removes a worn electrode cap from an electrode shank and mounts a new electrode cap on the shank. It would also be desirable to provide a resistance spot welding electrode cap changer which eliminates the need for manual labor required to replace or dress worn electrode caps. It would also be desirable to provide a resistance spot welding electrode changer in which the replacement of worn electrode caps with new caps is done in a short amount of time so as to minimize any production stoppages. It would also be desirable to provide a resistance spot welding electrode cap changer which can be used in conjunction with a robot, multi-axis welding apparatus.